The present invention relates to an ultrasonic diagnostic apparatus which transmits and receives an ultrasonic wave to and from an object to be examined so as to obtain an ultrasonic image of a region of interest and more particularly to an ultrasonic diagnostic apparatus which can drive a probe with a desired transmitting waveform to cause the probe to transmit an ultrasonic wave.
As shown in FIG. 1, a conventional ultrasonic diagnostic apparatus comprises a probe 1 having a great number of transducers arrayed to form multiple channels and adapted to transmit and receive an ultrasonic wave to and from an object to be examined, an ultrasonic wave transmitting circuit 6 for supplying an ultrasonic wave transmitting signal to the probe 1, a receiving beamforming circuit 3 for performing receiving beamforming by amplifying an ultrasonic receiving signal from the probe 1 and applying an amplified signal with a predetermined delay, an image processing circuit 4 for applying a predetermined processing to an output signal from the receiving beamforming circuit 3 so as to form an image signal, and an image display 5 for displaying the image signal from the image processing circuit 4.
In the ultrasonic wave transmitting circuit 6, its internal transmitting pulse generating circuit 7 generates an ultrasonic transmitting pulse for each transducer, the ultrasonic transmitting pulse is converted into a high voltage transmitting pulse by means of a driver circuit 8 and the high voltage transmitting pulse drives each channel, that is, each transducer or a pair of transducers, of the probe 1, thus causing the probe 1 to generate an ultrasonic wave. The receiving beamforming circuit 3 receives an echo signal reflected from an object to be examined and received by each transducer of the probe 1, amplifies the received echo signal, applies the amplified signal with a predetermined delay and adds signals of the respective channels together. In the image processing circuit 4, a resultant signal is subjected to a signal processing such as logarithmic compression and detection and converted into an image signal such as a television signal. The image display 5 displays the image signal as an ultrasonic image.
The driver circuit 8 included in the ultrasonic wave transmitting circuit 6 is exemplified in a circuit diagram of FIG. 2. In this circuit, an N-MOSFET, a P-MOSFET, a resistor R and a capacitor C are used and with a signal applied from a digital circuit of low voltage, for example, 5 V system to the input, a +V power supply is subjected to switching to generate a pulse of high voltage. The delivered high voltage pulse is sent to the probe 1 shown in FIG. 1 to drive the same. In order that an object to be examined can be prevented, for safety, from being applied with an excessively large ultrasonic wave, power of ultrasonic transmitting wave is controlled by changing voltage of the +V power supply to change the amplitude of the high voltage pulse to be delivered.
In the conventional ultrasonic diagnostic apparatus constructed as above, however, the FIG. 2 driver circuit 8 included in the ultrasonic wave transmitting circuit 6 generates a high voltage pulse for transmission by merely switching the high voltage +V power supply by means of switching elements such as the N-MOSFET and P-MOSFET and hence, the circuit 8 can generate only a high voltage pulse of fixed waveform which is of a substantially rectangular waveform, failing to generate a high voltage pulse of a desired waveform. Namely, the amplitude of each wave of an ultrasonic transmitting signal consisting of five waves as shown in FIG. 2 cannot be changed desirably. Further, in the circuit of FIG. 2, the amplitude of the high voltage pulse is changed by changing voltage of the high voltage +V power supply but the smoothing capacitor C for the +V power supply prevents voltage from changing at a high speed and so the amplitude cannot be changed at a high speed. For the above reasons, the transmitting wave cannot be controlled in amplitude, frequency, wave number and waveform independently for the respective channels of the probe 1 depending on the modes of monochromatic tomographic image and Doppler image and consequently, irregularity in sensitivity of transducer prevailing between one channel and another cannot be eliminated and besides individual channels cannot be weighted in terms of sensitivity. Further, since ultrasonic waves cannot be transmitted simultaneously in a plurality of scanning line directions, transmission of waves must be switched alternately in accordance with the modes of monochromatic tomographic image and Doppler image and the frame rate of an obtained ultrasonic image is degraded.